Airbag modules currently used in automobiles comprise an inflator for rapid generation of gas, activated upon rapid deceleration of the automobile as in a collision, and an airbag which will be deployed and inflated by gas from the inflator. The inflated airbag provides a cushion in front of a driver or passenger to arrest motion of the driver or passenger in a collision.
For an airbag restraint system to be effective, the inflator must produce a volume of gas very rapidly at pressure sufficient to deploy the airbag from a container in which the airbag is stored and to inflate the deployed airbag. This must all happen in the instant after a collision, to provide a restraining cushion in front of the driver or passenger as he is hurled forward or sideways by momentum. The cushion prevents him from striking a steering wheel or windshield or another part of the automobile. A problem in designing airbag modules is to generate enough gas fast enough and with sufficient pressure to deploy and inflate the airbag in that instant after collision, and without over inflating the airbag to a pressure that will make the airbag so hard that it becomes an unsafe object for the driver or passenger to strike. To deal with this problem designers have provided airbags which inflate in stages, first at initial pressure sufficient for rapid deployment and inflation, then in a second stage to relieve some of the pressure and provide a firm but softer cushion for safer restraint.
Several prior art designs have used airbags with variations on this two stage inflation principle. U.S. Pat. No. 5,290,061 described an airbag with a lower end portion of the bag folded inside a lower end portion of the bag prior to inflation. Upon inflation, the upper end portion was inflated and the folded in lower end portion would emerge and be inflated by expansion of compressed gas from the upper end portion. There was no means for restraining or controlling the emergence of the lower end portion from inside the upper end portion.
German Patent No. 2,944,319 described an airbag having two inner chambers. Before inflation, the two chambers were separated by a stitched seam across the center of the bag. In the first inflation stage, one of the chambers was inflated. When the first chamber had been fully inflated, gas pressure in the inflated first chamber caused the walls of that chamber to rip out the stitching, thus joining the second chamber to the first to provide added volume that would be filled by expansion of gas from the first chamber. When the airbag would be deployed, both of the two chambers would be deployed together. Until the uninflated second chamber was inflated, some time after deployment, it could freely flap within the passenger compartment, possibly causing harm by striking a passenger.